1. Field of the Invention
The present invention relates to an apparatus and a method for driving a touch panel, and more particularly, to an apparatus and a method of driving a touch panel that increases an analog-to-digital converting range for improving efficiency.
2. Description of the Related Art
In general, a resistive-type touch panel includes two resistive layers that face each other and are separated with a gap therebetween. One of the two resistive layers has a first pair of electrodes formed facing each other along edges of the resistive layer in a “X” direction, and the other resistive layer has a second pair of electrodes formed facing each other along edges of the other resistive layer in a “Y” direction. Consequently, an electric current is alternately applied through the electrodes in each of the resistive layers, thereby generating a voltage across each of the resistive layers. Thus, when a user presses the touch panel, the two resistive layer contact each other at the location where the user presses, thereby forming a contact point such that the coordinated position in term of the X-Y directions of the contact point can be detected by measuring the changes in voltages in the resistive layers. However, if a current is constantly applied to the resistive layers even during an input waiting period, i.e., when no contact point is made by a user, electric power is wasted during the input waiting period. Thus, in general, no electric current is applied to the touch panel during the input waiting period and the touch panel is designed to automatically detect when a contact point is made by a user.
FIG. 1 illustrates a circuit diagram of a resistive-type touch panel according to a related art, and FIG. 2 illustrates an equivalent circuit diagram of the touch panel of FIG. 1 when the touch panel is pressed by a user according to a related art. In FIG. 1, a pair of square-shaped resistive layers 10 and 12 are formed facing each other with a gap therebetween. The resistive layer 10 has a first electrode A1 and a second electrode A2 formed facing each other along edges of the resistive layer 10 in a “X” direction. The resistive layer 12 has a third electrode A3 and a fourth electrode A4 formed facing each other along edges of the resistive layer 12 in a “Y” direction. The second electrode A2 is connected via a first transistor Tr1 to a power source Vcc supplying a fixed voltage, and the fourth electrode A4 is connected via a second transistor Tr2 to the power source Vcc. In addition, the first electrode A1 is grounded via a third transistor Tr3 in parallel with a first resistor R1, and the third electrode A3 is grounded via a fourth transistor Tr4 in parallel with a second resistor R2. The voltage change of the third electrode A3 is referenced as a first positional detection signal Vx in the “X” direction and the voltage change of the first electrode A1 is referenced as a second positional detection signal Vy in the “Y” direction. Further, each of the transistors Tr1, Tr2, Tr3, and Tr4 is connected to a switching signal (not shown), such that during an input waiting period, all transistors Tr1, Tr2, Tr3, and Tr4 are turned OFF and the first electrode A1 is set to HIGH.
In FIG. 2, when a user presses the touch panel, the two resistive layers 10 and 12 contact each other at a contact point. Accordingly, the current flows to the ground through the resistive layers 10 and 12 and the second resistance R2, and the voltage level of the third electrode A3 also changes. Thus, a detection in changes of the voltage level of the third electrode A3 can be used as an indication on whether the touch panel has been pressed.
Subsequently, when the touch panel has been pressed, a switching control signal (not shown) is applied to bases of the first and third transistors Tr1 and Tr3 to periodically apply a power voltage from the power source Vcc between the electrodes A1 and A2, such that the first positional detection signal Vx can detect the contact point in the “X” direction at the third electrode A3. In addition, an inverted switching control signal (not shown) is applied to bases of the second and fourth transistors Tr2 and Tr4 to periodically apply the power voltage from the power source Vcc between the two electrodes A3 and A4, such that the second positional detection signal Vy can detect the contact point in the “Y” direction at the first electrode A1.
FIG. 3 illustrates a cross-sectional view of a resistive layer according to a related art. In FIG. 3, if a voltage is applied to the resistive layer, the voltage differs across the surface of the resistive layer. For instance, if the power source Vcc supplies a 5V voltage to a first electrode in a resistive layer, a measurement of the voltage taken at a point A adjacent to the first electrode is about 4.5–4.7V. In contrast, a measurement of the voltage taken at a point B adjacent to an opposing electrode is only about 0.3–0.5V. Thus, the voltage decreases across the surface of the resistive layer. Unfortunately, an active area of a touch panel is typically in a center region of the resistive layers. Since a reference voltage of an analog-to-digital converter is typically 5V, a detectable voltage range of about 0.4–4.6V can not be obtained in an active area of the touch panel, which is further away from the opposing electrodes. Thus, the performance of the related art touch panel has not been fully satisfactory.